Do you mean REAL 48 & 60 frames/sec on those cameras and cutting at REAL 24 fps in FCP?
Not 59.94 on that 60 FPS camera? Not 23.98/23.976 in FCP? It's important.

Let's assume you really DO mean 60 & 48, no fractional frame rates. If that's true, I have a neat trick for you. Perhaps two neat tricks.

First trick: grab a copy of Adobe After Effects. Import the 60 fps footage. Place it in a 24 fps AE comp. Render. The result: 24 fps footage that looks just fine... but it will be real-time motion, i.e. band playing fast. I suppose you could conform to 12 fps for a mild stop-motion sort of look with the band playing at the intended speed.

Second trick: put the 60 fps footage in a 48 fps After Effects comp, render, then conform to 24... but I don't know if it will make the nice, smooth slo-mo you seek. I haven't done the arithmetic on that one.

My advice on these After Effects tricks: do BOTH on a test clip. See which one works best for you. Then have at it for real.

First of all: DON'T CUT AT 24! F0RGET 24!
Cut at 23.976, or 23.98 in FCP-speak.

Next: get rid of the footage conformed to 24; re-conform it to 23.976.

The same tricks will still work, you can apply the same techniques, but you'll have to change the numbers for the frame rates:

60=59.94
48=47.952
24=23.976
12=11.988

...and in case you need it, 30=29.97

I once had a thick-skulled art director who didn't get fractional frame rates. He never understood that a frame rate is simply a measure of time. He kept seeing images of 0.976 frame in his head, as if someone has sliced off a tiny portion of the frame. He couldn't shake it.